When the first animal and human defibrillations were reported with both internal and external electrodes, the electrical waveform utilized was a 60 cycles/second (Hz) sinusoidal waveform. This electrical shock was obtained by modifying the available voltage, typically 110 V(rms), such as by stepping up or stepping down the voltage. Durations were approximately in the range of 100 to 150 milliseconds (msec). Disadvantages to this methodology included: the defibrillator was very large and not easily portable; the defibrillator had to be plugged into the wall and, therefore, the patients had to be in the hospital; and there was a large current draw during the shock, which blew fuses and dimmed other lights on the circuit.
In the 1960""s Edmark and Lown independently developed new waveforms for defibrillation which are called RLC waveforms. These waveforms are generated with a circuit containing a capacitor (C), an inductor (L), and a resistance (R). Advantages to the use of these waveforms included: the defibrillator was small and portable; it could be powered by a battery and used out of the hospital; and it did not draw huge amounts of current. These waveforms quickly became the standard for transthoracic defibrillation, and are still the industry standard for transthoracic defibrillation today.
When the implantable cardioverter/defibrillator (ICD) was developed in the 1970s [Schuder et al, Trans ASAIO, 16:207-12], the waveform of choice was the monophasic truncated exponential (MTE) as this waveform could be generated without an inductor (which could not be miniaturized for implantable devices). The MTE waveform was pioneered by the laboratory at University of Missouri [Gold et al, Circ 56:745-50, 1977] and was incorporated into most ICDs for clinical use for the first decade of their use.
In a series of papers in the early 1980s, the laboratory at the University of Missouri (xe2x80x9cMU Labxe2x80x9d) pioneered a new class of waveforms for electrical ventricular defibrillation, called bidirectional or biphasic waveforms. The MU Lab demonstrated that if one were to reverse the polarity of an MTE waveform for the second half of the duration to yield a biphasic truncated exponential waveform (BTE), that one could dramatically improve the efficacy of defibrillation. The MU Lab studies covered the cases where the second phase amplitude was equal to the first phase amplitude and constant; where the second phase amplitude was smaller than the first phase amplitude and constant; and where the first and second phase amplitudes were allowed to decay exponentially and the second phase amplitude was either smaller than or equal to the first phase amplitude. Some of these waveforms studied were the first use of single capacitor waveforms (waveforms that could be generated by switching the polarity of a single capacitor) for defibrillation. These early studies utilized both internal and external electrodes.
The early studies from the MU lab arbitrarily set the first phase duration equal to the second phase duration. In 1987, Dixon et al published a paper, which found that if the first phase was longer than the second phase, that one could improve the efficacy of defibrillation over the case where the second phase duration was longer than the first phase duration [Dixon et al, Circ 76:1176-84]. The company that sponsored this research (Intermedics, Inc.) subsequently received U.S. Pat. No. 4,821,723 relating to this variation of the biphasic waveform. Biphasic truncated exponential (BTE) waveforms are now the industry standard for ICDs and also for implantable atrial defibrillators (IADs).
Several theories have been put forward, in an effort to understand why biphasic waveforms are generally more effective for electrical defibrillation than are monophasic waveforms. Understanding the mechanism of biphasic waveform superiority will possibly allow the design of even better waveforms for the next generation of defibrillators. The dominant theory in the field is currently a group of theories which can collectively be called RC circuit model theories. These theories have the common feature of modeling the response of the heart to a defibrillation shock, as the response of a resistor-capacitor (RC) circuit to that same shock. These theories also share the view that defibrillation efficacy is determined by the maximal capacitor voltage (model response) and the final capacitor voltage (model response). Taken as a group, these theories have led investigators to postulate optimal BTE waveforms for both internal and external defibrillation. As an example, a 1997 PCT publication xe2x80x9cExternal defibrillator having low capacitance and small time constantxe2x80x9d [WO 97/38754] relates to a BTE according to one version of the RC circuit model theory. Other theories of defibrillation have similarly led to different optimal waveform designs.
There are three different phenomena where electrotherapy shocks such as these are useful. The three phenomena are ventricular defibrillation, atrial defibrillation, and cardioversion; which are the treatment by electrical shock of ventricular fibrillation, atrial fibrillation, and atrial and/or ventricular tachycardia. Each of these three phenomena can be accomplished with electrodes that are external to the body, or with electrodes that are implanted either permanently or temporarily in the body. The state of the art treatment for all six combinations of these conditions and electrodes is presently some variation of the biphasic waveform. Currently, the same device is typically used for both ventricular defibrillation and cardioversion. For example, CPI Guidant calls their internal defibrillator an Automatic Implantable Cardioverter Defibrillator, which implies a single device with two functions.
The efficacy of these cardioverter and/or defibrillator devices in practice, is determined by the electrical waveform generated, and by the way the device compensates for variations in the patients to which it is applied. Specifically, the electrical impedance varies from patient to patient, and over time within a patient. This variation is much larger in magnitude when external electrodes are used, than when internal electrodes are used. Consequently, compensation for this variation in impedance is more critical in external defibrillators than in implantable defibrillators. Some devices use a passive impedance compensation strategy, whereby changes in impedance cause waveform changes without active intervention. Other devices actively compensate for impedance variation by measuring electrical parameters before or during the discharge such as capacitor voltage, patient impedance, or current flow; and modifying the electrical waveform based on these measurements.
For external ventricular defibrillation and cardioversion, the biphasic waveform of the Heartstream Inc.""s FORERUNNER(copyright) device is representative, and this device uses an active impedance compensation strategy. For an average impedance patient, it delivers a single capacitor BTE waveform with a 7 msec first phase and a 5 msec second phase, and uses a 100 microfarad capacitor. In response to variations in patient impedance, this device changes the durations of the two phases, the overall duration of the waveform, and the ratio of the durations of the two phases. The FIRSTSAVE(copyright) AED device made by SurVivaLink Corporation also delivers a biphasic waveform, and optimizes the waveform in terms of a charge-burping theory of defibrillation with active compensation for variations in impedance. Another alternative by Zoll Medical Corporation is the defibrillator waveform which has a saw-tooth (roughly constant current for all impedances) first phase, followed by a decaying exponential second phase. The biphasic waveform of the LIFEPAK(copyright) device by Physio-Control Corporation differs by using a longer time constant, and therefore a larger capacitance (about 300 microfarads). This device also uses an active impedance compensation strategy. External ventricular defibrillators on the market today deliver either a monophasic waveform (Edmark or truncated exponential) or some variation of the biphasic waveform, usually with the second phase shorter in duration and smaller in amplitude than the first phase. With applications such as the automatic external defibrillator (AED), it is very desirable to design a defibrillator that will work well with all impedance patients. In this application, simplicity of the circuitry is also an advantage, to reduce the cost of the devices, and promote more widespread availability.
For internal ventricular defibrillation and cardioversion, there are several companies that currently make devices, and most (if not all) of these devices deliver some version of a biphasic waveform. Many studies have been published, attempting to optimize the biphasic waveform for internal ventricular defibrillation. It is important to optimize the waveform for this application to avoid wasting energy, which will deplete the device battery prematurely. In addition, it is desirable to program the output of the implantable defibrillator to the lowest output level that will reliably defibrillate. This again avoids wasting battery life, as well as minimizing the detrimental effects of delivering too much energy to the heart. One patent that covers the biphasic waveform is the Baker et al patent, U.S. Pat. No. 4,821,723.
Another relevant patent is U.S. Pat. No. 4,637,397 on a triphasic waveform for defibrillation. In this patent, the figures show a small amplitude first phase, a larger amplitude second phase, and a very low amplitude final phase. This is in keeping with the inventor""s theory that the first phase conditions the heart, the second phase defibrillates, and the final phase heals the heart. There have also been many other studies of a multitude of other waveforms, including many different multiphasic waveforms, most of which are delivered to two different pairs of electrodes in sequence. But none of these studies has resulted in a waveform that is clearly superior to the biphasic waveform for electrical ventricular defibrillation.
There have also been papers published of studies optimizing the biphasic waveform for atrial defibrillation. The first such paper (Cooper et al, Circulation 87:1673-86, 1993) found that the optimal BTE waveform was a single capacitor waveform that had a 3 msec first phase and a 3 msec second phase. Studies in patients, however reported that this waveform caused too much pain, and that a 6 msec plus 6 msec biphasic waveform allowed a reduction in peak current and also reduced the pain associated with the shock. However, this waveform is still associated with significant pain that has impeded its widespread clinical acceptance. Much of the current research in this area is concentrating on strategies to prevent AF, and pacing strategies to correct AF, without needing a defibrillation shock. Many physicians are reluctant to implant present atrial defibrillators, as the patients do not tolerate the pain associated with the shocks. A method that could electrically treat AF without pain would be a very welcome addition for therapy with implantable devices.
Another major application in need, is an external device connected to internal electrodes, for use (for example) in the intensive care unit with patients after heart surgery. As many as 40% of these patients will experience AF in about the first week after surgery. Not wanting to electrically shock these patients, they are typically kept in the hospital an extra day or two, until the AF resolves. After the first week, the increased incidence of AF disappears, and therefore an implanted unit is not needed. Another use of such a device could be in the electrophysiology laboratory, where a temporary catheter would be inserted to treat chronic atrial fibrillation. BTE waveforms have been tried in this application, but once again it was found that the pain associated with the shock was not acceptable to patients. A less painful, or painless therapy would be a very welcome addition to the treatment options for these patients.
From circuit theory, we know that if one increases the duration of a defibrillator shock, that the amplitude of the shock can be reduced and still deliver the same energy. Studies showed that biphasic waveforms with an overall duration of 12 msec were less painful for atrial defibrillation than those with an overall duration of 6 msec, and that they required less peak current. This led many to conclude that high peak currents were causing the pain of atrial defibrillation. Studies were performed to further increase the duration of the biphasic waveform, hoping to further reduce the peak current amplitude required for atrial defibrillation, and further reduce the associated pain. However, they reported, and recent studies confirmed, that the biphasic waveform for atrial defibrillation loses its efficacy as the duration is increased. This loss of efficacy is very pronounced with atrial defibrillation, as compared to the same waveforms used for ventricular defibrillation. Optimization of the biphasic waveform has not solved the problem of pain associated with an atrial defibrillation shock.
The effectiveness of an electrical defibrillation shock has been known for many years to be dependent on the shape of the electrical waveform. In other words, the manner in which current, or voltage, changes with time is critical to determining whether the electrical shock will successfully defibrillate the heart. This observation has naturally led many to conclude that the key to understanding the mechanism of electrical defibrillation would be found in studies of electrical waveforms in the time domain, wherein shock intensity is given as a function of time. Many of the theories of biphasic waveform superiority to monophasic waveforms postulate a two step process, wherein the first phase has one function and the second phase another. And the theory of the triphasic waveform of Jones also postulates a process that is sequential in time. However, these studies in the time domain have not solved some of the major challenges remaining in this field.
There exists, however, a parallel domain, the frequency domain, into which any electrical waveform from the time domain can be transformed. This transformation, which can be performed with the Fourier Transform, is a reversible transformation which means that all the information present in the time domain is also present in the frequency domain. Further, the inventor has found that there exists frequency ranges in the frequency domain representation, wherein the delivery of energy is associated with increased efficacy for electrical defibrillation, and that these optimal ranges may be different for the different applications of cardioversion and defibrillation shocks. It follows from this that there are frequency ranges in the frequency domain representation, wherein the delivery of energy is not associated with increased efficacy for electrical defibrillation. This invention optimizes defibrillation waveforms by maximizing the amount of energy delivered in the beneficial frequencies, and minimizing the amount of energy delivered in the other frequencies.
In addition, the full description of the Fourier transform requires information on the phase angle of the transform, and the inventor has found that defibrillation efficacy is also dependent on phase angle. Selecting a waveform so that the delivered energy is in optimal frequency ranges is one alternative condition of optimizing the waveforms in the frequency domain; selecting a waveform so that a dominant frequency of the waveform in the frequency domain is in a preselected range is another alternative condition of optimizing the waveforms in the frequency domain; and selecting the optimal phase angle of the transform of the waveform in the frequency domain is yet another alternative condition of optimizing the waveforms in the frequency domain.
The present invention improves the effectiveness of shocks used for electrical defibrillation or cardioversion of either the atria or ventricles, by maximizing the amount of energy delivered in an optimal frequency range and with the optimal phase angle. (As used herein, defibrillation includes cardioversion.) There are several advantages to the present invention over currently available technology, which are specific to the various applications of this invention.
Transthoracic Defibrillationxe2x80x94This invention can be applied to transthoracic defibrillation of humans, in which case the clinical device will be presented with widely varying patient impedance levels. Therefore, this invention is applied to design the waveform generated for a typical, or average impedance patient, and is also applied to design the impedance compensation strategy. The resultant waveform has the advantage of responding to different impedances by maximizing the amount of energy delivered in the optimal frequency range for ventricular defibrillation. The present invention in this application yields an electrical waveform for transthoracic ventricular defibrillation that has been demonstrated in animal studies to be more effective than the industry standard Edmark waveform at all simulated patient impedances. And this waveform has been demonstrated to be as effective as a state of the art biphasic waveform in simulated low and average impedance patients, and more effective than this prior art BTE waveform when the patient impedance is high. This waveform is also simpler to generate, and could lead to smaller, less expensive external defibrillators. When this invention is applied to external atrial defibrillation, the resultant optimal waveform may be different than the waveform for external ventricular defibrillation, as the optimal frequency range for atrial defibrillation appears in animal studies to be different than the optimal frequency range for ventricular defibrillation.
Internal Defibrillationxe2x80x94When applied to internal atrial or ventricular defibrillation, the present invention again has the advantage of optimizing the electrical waveform for each application, based on the optimal frequency ranges for that application. The present invention will also allow the generation of an electrical waveform for internal atrial defibrillation that is as effective as the prior art at varying patient impedances and that will defibrillate with appreciably less peak current than the prior art. This will likely translate into a waveform that causes less pain, when applied to humans in atrial fibrillation. In addition, this invention will allow the design of a waveform that is less effective at stimulating pain receptors, to cause less pain when applied to patients in atrial fibrillation. The present invention may also be applied to internal ventricular defibrillation, wherein by maximizing the amount of energy delivered in the optimal frequency range, one can improve the efficacy of the waveform across different patient impedances.
In one form, the invention comprises an apparatus for treating fibrillation or tachycardia comprising a discharging energy source, two electrodes adapted to make electrical contact with a patient, a connecting mechanism forming an electrical circuit between the energy source and the electrodes and a controller. The controller operates the connecting mechanism to deliver pulses of electrical energy from the energy source to the electrodes having a multiphasic waveform having three or more pulses optimized in the frequency domain.
In another form, the invention comprises a method of generating a waveform for treating fibrillation or tachycardia in a patient comprising discharging an energy source across electrodes in contact with the patient to deliver electrical energy from the energy source to the electrodes having a multiphasic waveform and optimizing the waveform in the frequency domain.
In yet another form, the invention comprises a discharging energy source, two electrodes adapted to make electrical contact with a patient, a connecting mechanism forming an electrical circuit between the energy source and the electrodes and a controller. The controller operates the connecting mechanism to deliver electrical energy from the energy source to the electrodes having a particular one of a plurality of waveforms, each of which is optimized in the frequency domain.
In yet another form, the invention comprises a signal for treating fibrillation or tachycardia comprising a multiphasic waveform containing three or more pulses optimized in the frequency domain.
Other objects and features will be in part apparent and in part pointed out hereinafter.